Concept Group 1. Concepts Applying to Fundamental Principles of Toxicology

Subgroup A – Exposure and Toxicity

1.1 Acute and Chronic 14
1.2 Interaction 18
1.3 Dose 21
1.4 Adverse Effect and Toxicity 22
1.5 Toxicity Classification, Labelling and Material Safety Data Sheets 26
1.6 Terms Applied to Toxic Substances 35

1.1 Acute and Chronic

A. Acute

acute

1 Of short duration, in relation to exposure or effect. In experimental toxicology, acute refers to studies where dosing is either single or limited to one day, although the total study duration may extend to two weeks.

2 In clinical medicine, sudden and severe, having a rapid onset. Antonym: chronic.

acute effect

Effect of finite duration occurring rapidly (usually in the first 24 h or up to 14 d) following a single dose or short exposure to a substance or radiation.

acute exposure

Exposure of short duration.

Antonym: chronic exposure.

acute toxicity

1 Adverse effects of finite duration occurring within a short time (up to 14 d) after administration of a single dose (or exposure to a given concentration) of a test substance or after multiple doses (exposures), usually within 24 h of a starting point (which may be exposure to the toxicant, or loss of reserve capacity, or developmental change, etc.).

2 Ability of a substance to cause adverse effects within a short time of dosing or exposure.

Antonym: chronic toxicity.

In toxicology, 'acute' is a word that is used in combination with exposure, toxicity and effect. Acute exposure is a single or very short-lasting dosing by any route. Talking about acute toxicity addresses adverse effects (further discussed below), i.e. harmful effects, unwanted negative effects that occur immediately after or within a short time after administration of a single dose of a substance, or following short exposure or concurrently with continuous exposure, or recurrently following shortly after multiple doses. 'Short' implies a time of 24 h or less. Some effects considered to be acute can occur up to as long as 96 h after exposure. Uraemia can be an acute effect, but it takes almost 96 h to see such an outcome. In toxicity testing, it is most important to be aware of this in order not to draw any false conclusions from animal studies with agents that cause such an acute effect. Acute effects usually occur or develop rapidly after a single exposure. However, acute effects can also appear immediately after, or during, repeated or prolonged exposure.

Acute Toxicity. Historically, an important aspect of acute toxicity has been the identification of the lethal dose or exposure that kills an organism after a short exposure or a single dose. This has been established by a test in which selected organisms are exposed to a series of increasing dose levels until a dose is reached at which all the organisms die. For regulatory purposes, to permit extrapolation to humans, it is usually performed with at least two mammalian species. From such tests, the LD50 for the test species has been derived and used for the classification of the toxicity of chemicals to humans. Such tests involved killing large numbers of animals to obtain a toxicity classification based on lethality. However, this classification tells us nothing about sublethal effects such as immunotoxicity or teratogenicity. This situation was clearly unsatisfactory and so acute toxicity testing is now designed in such a way as to obtain maximum information about all aspects of acute toxicity using the minimum number of animals.

In Europe, classification of new chemicals for toxicity is no longer based on the LD50. The tests used for this purpose are based on survival rather than on lethality. For example, the method of fixed-dose testing is usually limited to a maximum dose of 500 mg per kg body weight. If five males and five females exposed to a dose of this magnitude survive with no evidence of toxicity, the chemical tested need not be classified as toxic. Toxicity classifications based on this approach can provide a similar classification of toxicity to the old LD50 system but with a huge reduction both in the number of animals used and in animal suffering compared to the traditional LD50 tests. Another approach to reducing the numbers of animals used is the up-and-down procedure, which also produces a value approximating to the LD50. This procedure uses sequential dosing together with sophisticated computational methods. It provides a point estimate of the LD50 while achieving significant reductions in animal use.

Since chronic toxicity testing (see below) is expensive and labour-intensive, there is a great need to replace it where possible with shorter-term predictive acute tests and early identification of biomarkers of toxicity. This has been possible to some extent with carcinogenicity. In the past, a cancer study was designed to expose animals to the toxicant and to follow the animals during their lifetime. Each animal upon death was examined for occurrence and localization of tumours in the body. Since it is expensive to maintain animals over long periods, the need for new tools to identify carcinogens is clear. Many cancers start with mutations or chromosome damage, and this can be assessed with short-term tests such as the Ames test or the host-mediated (Legator) test. The Ames test is based on reversal of a point mutation in a Salmonella strain, which makes it unable to synthesize the amino acid histidine. Back-mutation can be detected by growth of the bacteria in a histamine-depleted medium. Rat liver microsomes are included in the test medium to simulate the metabolic activation of organic compounds that may take place in the intact animal. The host- mediated test looks for chromosome changes in vitro and (or) in vivo, including chromosome breaks and sister chromatid exchanges, in microbial cells introduced (e.g. by intravenous injection) into a host animal. The host animal receives the test compound orally and therefore acts as a source of chemical metabolism, distribution and excretion. Another whole animal test involves looking for the production of micronuclei in animals exposed to possible carcinogens. The micronucleus test is less sensitive than bacterial tests but is a more realistic measure of likely chromosomal damage in mammals at risk.

It is also possible to test quickly for mutagenicity and the possibility of associated carcinogenicity by adding suspect substances to cell cultures and looking for chromosome damage and cell transformation. Another approach to carcinogenicity testing is to apply the substances to tissues in culture and/or to genetically compatible transplants and similarly assess the changes...